Added BENCH/TEST_PRNG, replacing other ad-hoc sources of randomness

When you add a function to every benchmark suite, you know if should
probably be provided by the benchmark runner itself. That being said,
randomness in tests/benchmarks is a bit tricky because it needs to be
strictly controlled and reproducible.

No global state is used, allowing tests/benches to maintain multiple
randomness stream which can be useful for checking results during a run.

There's an argument for having global prng state in that the prng could
be preserved across power-loss, but I have yet to see a use for this,
and it would add a significant requirement to any future test/bench runner.

tests/test_orphans.toml

@@ -77,13 +77,13 @@ code = '''
         lfs_mount(&lfs, cfg) => 0;
     }
 
-    srand(1);
+    uint32_t prng = 1;
     const char alpha[] = "abcdefghijklmnopqrstuvwxyz";
     for (unsigned i = 0; i < CYCLES; i++) {
         // create random path
         char full_path[256];
         for (unsigned d = 0; d < DEPTH; d++) {
-            sprintf(&full_path[2*d], "/%c", alpha[rand() % FILES]);
+            sprintf(&full_path[2*d], "/%c", alpha[TEST_PRNG(&prng) % FILES]);
         }
 
         // if it does not exist, we create it, else we destroy